1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and a recording medium, for converting print data into bitmap data to print the data.
2. Description of Related Art
In recent years, there have been widespread image forming apparatuses such as a printer and a multi function peripheral (MFP) for once converting print data received from an external apparatus such as a personal computer (PC) into data having an intermediate format (hereinafter, referred to as intermediate data), rasterizing the intermediate data to generate bitmap data, and printing the data. In such an image forming apparatus, one page is divided into a plurality of bands for each predetermined number of lines, and the rasterizing processing is performed.
For example, there has been proposed a technology in which a sub-band memory is provided in an image expanding device using alternately two band memories for expanding an image and in which when an image is complicated and takes time to be expanded, the image is rasterized in advance (refer to JP-Tokukai-H09-001873A). When the intermediate data is generated, it is determined whether the image is expanded to the sub-band memory or not.
FIG. 13 shows a memory size required to hold bitmap data for one band. As shown in FIG. 13, when the width of one band is W and the number of lines of one band is H, a memory size of (W×H×4) [Bytes] for one band is required for four planes of CMYK in the case of a color image forming apparatus. In general, the image forming apparatus comprises a memory for two bands, which is used as a memory area for the rasterizing processing and as a memory area for transferring the rasterized data to a printer engine.
Incidentally, as image forming apparatuses capable of directly printing a portable document format (PDF) data have been being increased, drawing functions of the PDF have also been more complicated. As one of the drawing functions of the PDF, there is a transparency function.
As shown in FIG. 14, the transparency function of the PDF is a function to perform processing (hereinafter, referred to as transparency processing) for allowing newly added source data (hereinafter, referred to as Src data) to be transparent on destination data (hereinafter, referred to as Dest data) which is a background, according to alpha channel data indicating the opacity. The alpha channel data may be one to designate the opacity for each pixel, or may be one to have the fixed opacity.
In the case of performing the transparency processing, as shown in FIG. 15, it is necessary to hold all of the Dest data, the Src data, the alpha channel data and transparency processing result data on the memory. Accordingly, the enormous memory size becomes necessary. In the case of the color image forming apparatus, the Dest data, the Src data, and the transparency processing result data have four planes of the CMYK, respectively. Accordingly, the following memory size is further required:{(W×H×4)+(W×H×4)+(W×H)+(W×H×4)} [Byte]
For example, as shown in FIG. 16A, when print data (width: 4960 pixels; the number of lines: 7000 lines) having a resolution of 600 dots per inch (dpi) on a sheet of the A4 size is processed for one band having the number of lines of 256, the memory size required to hold the bitmap data for one band is as follows:(4960×256×4) [Byte]=4.8 [MByte]for four planes of the CMYK.
In the case of performing the transparency processing, as shown in FIG. 16B, the following memory size is further required:{(4960×256×4) for Dest data} [Byte]{(4960×256×4) for Src data} [Byte]{(4960×256) for alpha channel data} [Byte]{(4960′256′4) for transparency processing result data}[Byte]=15.7 [MByte]
In general, even though a rasterizing area for two bands is ensured, the memory size of 9.7 [MByte] is only required. However, in the case of performing the transparency processing, the memory size of 15.7 [MByte] becomes additionally necessary at the minimum. Moreover, in the case of further using the transparency processing result data obtained by the transparency processing as the Dest data or the Src data, a larger memory size is required. Hence, it is necessary to ensure a large memory size in order to deal with the transparency processing.
However, in the image forming apparatus such as the printer, the MFT or the like, the size of memory which can be mounted on the apparatus is limited. Accordingly, when the print data includes data (hereinafter, referred to as transparency processing data) for use in the transparency processing, such as the Dest data, the Src data, and the alpha channel data, a shortage of the memory may occur.